In U.S. Pat. No. 4,264,978 which issued on Apr. 28, 1981, apparatus is disclosed for locating the presence of an audio surveillance device, commonly referred to as a "bug." This apparatus includes a conventional scanner for detecting carrier (or sub-carrier) frequencies and sequentially locking onto these frequencies. When a frequency is detected which carries no modulation, a single acoustic pulse is generated by the apparatus, and the pulse is propagated through the air so as to be picked up and retransmitted as a modulated signal by the transmitter of any bug in the vicinity. If the frequency to which the scanner is then locked is that of the bug's transmitter, the surveillance apparatus demodulates the signal received by it to produce an electrical pulse. Circuitry is provided within the apparatus which effectively measures the time between the transmission of the acoustic pulse and its return in the form of an electromagnetic signal so as to produce an indication of the distance between the detection apparatus and the bug.
While the apparatus disclosed in the aforesaid patent is quite reliable, it is unable to automatically determine, prior to the transmission of an acoustic pulse, whether the scanner is locked onto the carrier frequency of a bug or that of some other transmitter which, at the time, does not happen to be producing a modulated output. This can create a problem, for if there is a bug present but the detecting apparatus is locked onto another unmodulated carrier frequency, the propagation of the acoustic pulse will be picked up by the surveillance device. If this occurs repeatedly, it is likely that the operator of the device will be aware that efforts are being made to detect its presence, and he then can frustrate such efforts by disabling the bug's transmitter. Furthermore, once the person operating the bug is aware that a search effort is under way, the party under surveillance is denied the advantage of being able to deliberately disseminate inaccurate information with the likelihood of it being considered reliable by the bug's operator.